Pervaporation is a process for the separation of liquid mixtures by partial vaporization through a membrane. The separation process has two steps: first, one component of the mixture permeates away from the mixture through the membrane (the escaping component is called the permeate, and the remaining mixture is called the retentate or concentrate); and second, the permeate evaporates away from the membrane. Pervaporation, Wikipedia (Mar. 10, 2010).
The efficacy of the pervaporation membrane may be determined by the membrane's selectivity (expressed as separation factor) and productivity (expressed as flux). Flux refers to the rate of flow or transfer of permeate from the mixture to vapor, and denotes a quantity of permeate that crosses a unit of area of a given surface in a unit of time. Separation factor refers to the membrane's ability to selectively remove more of one mixture component than the other component(s) of the mixture. Productivity and selectivity are membrane-dependent. Membrane technology, © 1998-2009 Lenntech Water Treatment & Purification Holding B.V., Delft, the Netherlands (www.lenntech.com).
US Publication 2011/0266222 discloses a method to dehydrate organic liquid (e.g., ethylene glycol, EG) by pervaporation using a PBI permselective hollow fiber. The PBI permselective layer of the hollow fiber is not chemically modified to make it acid resistant.
The dehydration of acidic solvents is an important commercial operation. One acidic solvent, acetic acid, is among the top 50 chemicals based upon production quantity. Currently, the dehydration of acidic solvents, e.g., acetic acid, is carried out by a binary distillation. This separation method, however, is difficult as acetic acid and water have very close volatilities. As such, more energy is required to achieve acetic acid with purity higher than 95 wt % due to the need for greater reflux and a larger distillation column with many stages. In view of these disadvantages of distillation, research emphasis has been placed on the pervaporation dehydration of acetic acid. More specifically, research has focused on developing a membrane that gives a reasonable flux and has a good separation factor. In pervaporative dehydration, a significant number of pervaporation dehydration membranes are made from cross-linked polyvinyl alcohol (PVA), chitosan, and cellulose acetate.
Accordingly, there is a need for new and better pervaporation membranes, particularly, pervaporation membranes for dehydration, and the dehydration of acidic solvents, such as acetic acid.